The present invention relates to the application of coatings to the surface of semicrystalline polymeric films. A process is described for treating at least one surface of the polymer film so as to enhance its ability to bond to coatings applied to that surface. 2. Background of the Art
some polymeric surfaces have been difficult to use as carrier or barrier layers because they resist adhesion to other materials. This is particularly true of semicrystalline polymers such as polyester, especially poly(ethylene terephthalate). In order to provide articles of certain materials carried on a polyester base, it has been necessary to provide one or more intermediate layers on the polyester before the single desired material can be applied. For example, in the photographic area, polyester substrates are provided with a primer layer (e.g., polymers or copolymers based on polyvinylidene chloride), a thin gelatin layer, and then the silver halide/gelatin photographic emulsion layer is applied to the coated substrate This is an expensive process, and once the polyester has been coated, it becomes difficult to salvage or recycle any waste material or poorly prepared material.
The effects of actinic radiation on the degradation of polymer surfaces have been studied for many years. Prior to about 1970, this work was done with low intensity photolamps at wavelengths greater than 220 nanometers (nm). Numerous papers are available in the literature, typical of which are Day and Wiles, Journal of Applied Polymer Science, 16 175 (1972), and Blais, Day and Wiles, Journal of Applied Polymer Science, 17 p. 1895 (1973).
Between 1970 and 1980 the effects on polymer surfaces of ultra-violet (UV) lamps with wavelengths less than 220 nm were studied for lithography and surface modification purposes. Such studies are exemplified by Mimura et al., Japanese Journal of Applied Physics, 17 541 (1978). This work illustrates that long exposure times and high energies are required to cause photo-etching when UV lamps are used. U.S. Pat. No. 3,978,341 (Hoell) teaches an apparatus for exposing polymeric contact lenses to a spark discharge producing 83 nm to 133.5 nm U.V. radiation to improve the wettability and adhesiveness of the lenses.
In 1975 the excimer laser was introduced. An excimer laser is an excited dimer laser where two normally non-reactive gases (for example Krypton, Kr, and Fluorine, F.sub. 2) are exposed to an electrical discharge. One of the gases (Kr) is energized into an excited state (Kr*) in which it can combine with the other gas (F.sub. 2) to form an excited complex (KrF*). This complex gives off a photon and drops to an unexcited state which, being unstable, immediately disassociates to the original gases (Kr and F.sub. 2) and the process is repeated. The released photon is the laser output. The uniqueness of the excimer laser is its high efficiency in producing short wavelength (UV) light and its short pulse width. These attributes make the excimer laser useful for industrial applications. Kawamura et al., Applied Physics Letters, 40 374 (1982) reported the use of a KrF excimer laser (248 nm wavelengths) to photo-etch polymethyl methacrylate (PMMA), an amorphous polymer used in preparing photolithography resists for semiconductor fabrication.
U.S. Pat. No. 4,414,059 (Blum, Brown and Srinivasan) disclosed a technique for the manufacture of microelectronic devices utilizing ablative photodecomposition of lithography resist amorphous polymers at wavelengths less than 220 nm and power densities sufficient to cause polymer chain fragmentation and immediate escape of the fragmented portions. The photodecomposition leaves an etched surface. The authors found that using an argon/fluorine (ArF) excimer laser emitting at 193 nm with a 12 nanosecond pulse width, a threshold for ablatively photo decomposing poly(methylmethacrylate) resist material occurs at a fluence of about 10-12 mJ/cm.sup.2 /pulse. It is stated that large amounts of energy, greater than the threshold amount, must be applied before ablation will occur. The energy used must be (1) sufficiently great and (2) applied in a very short amount of time to produce ablative photodecomposition.
U.S. Pat. No. 4,417,948 (Mayne-Banton and Srinivasan) and a related publication, Srinivasan and Leigh, Journal American Chemical Society, 104 6784 (1982) teach a method of UV photo etching poly(ethylene terephthalate) (PET). In these publications the authors indicate the mechanism of photo etching to be one of chain scission or bond breaking of surface polymer molecules by the high energy UV. Bond breaking continues in the presence of irradiation and the smaller units continue to absorb radiation and break into still smaller units until the end products vaporize and carry away any excess photon energy. This process results in small particles being ablated away, and various gases being evolved. The remaining surface material comprises molecules of low molecular weight (oligomers). Examining the PET repeating unit and the author'sclaim of bond scission, it is believed that the following occurs: ##STR1## Indeed, in the Journal of the American Chemical Society article, the authors analyze for benzene and start detecting it at about the threshold for photodecomposition for PET; i.e., about 20mJ/cm.sup.2 /pulse at 193 nm. The authors also indicate that the photo etch process is accelerated in the presence of oxygen which seals the ends of the broken chain'sfragments and prevents recombination of these fragments.
Srinivasan, Journal of the Vacuum Society, B1, 923 (1983) reports the results of ablative photodecomposition of organic polymers through a 0.048 cm diameter mask and states that a threshold exists for the onset of ablation and, for PMMA, that the threshold is 10mJ/cm.sup.2 pulse. He then goes on to state that one pulse at 16mJ/cm.sup.2 gave an etch mark on PMMA while 50 pulses at 4mJ/cm.sup.2 /pulse left no detectable etch marks. For PET and polyimide, the threshold began at about 30mJ/cm.sup.2 /pulse. However, for a satisfactory etch pattern the optimum fluence ranged from 100 to 350 mJ/cm.sup.2 /pulse.
In Srinivasan and Lazare, Polymer, 26, 1297 (1985) Conference Issue, the authors' report the photo etching of 6.times.12 mm samples of PET, PMMA and polyimide polymers with both continuous radiation at 185 nm from UV lamps and pulsed radiation at 193 nm from an excimer laser. The use of continuous low energy UV lamps caused photo oxidation of the polymer surface with a resultant increased oxygen to carbon ratio (O/C ratio) as determined by X-ray photoelectron spectroscopy (XPS) equipment, while the use of a pulsed high energy excimer laser, which produces chain scission in and ablation of the polymer surface, resulted in a lower O/C ratio as determined by XPS. The authors then go on to say "It may be pointed out that ablative photodecomposition is not exactly a method for the modification of a polymer surface at an atomic level since it totally eliminates the atoms at the surface and creates a fresh surface."
U.S. Pat. No. 4,568,632 (Blum, Holloway and Srinivasan) claims a method for photo etching polyimides. The process described uses a pulsed excimer laser at 193 nm. The stated incident energy required for photo ablation is much higher for polyimide than for PET. The value for the laser fluence threshold of PET was reported as about 30 mJ/cm.sup.2 /pulse while for polyimide it was reported as about 50 mJ/cm.sup.2 /pulse. An operative level was noted as about 50-100 mJ/cm.sup.2 /pulse for PET and 100-300 mJ/cm.sup.2 /pulse for polyimide. The etch rate found for PET was 1000 Angstroms for a fluence of 100-300 mJ/cm.sup.2 /pulse and for the polyimide was 750 Angstroms for 350 mJ/cm.sup.2 /pulse.
Lazare and Srinivasan, Journal Physical Chemistry, 90, 2124 (1986) report on the study of surface properties of PET which have been modified by either pulsed UV laser radiation or continuous UV lamp radiation. The authors report on the high fluence ablation of PET as follows: (1) the PET irradiated surface is a layer of low molecular weight material, (2) the surface has a rough chemically homogeneous texture, (3) the surface has a high chemical reactivity characteristic of oligomers, and (4) the surface could be removed by washing in acetone. Since extremely low molecular weight fragments (oligomers) of PET are soluble in acetone, the authors assert this removal of the treated surface is indicative of the presence of low molecular weight material on the surface. The authors also report that the low intensity UV lamp treated PET surfaces would not wash off with acetone. This later article reports thresholds for ablation of PET at about 30-40 mJ/cm.sup.2 /pulse.
Japanese Patent Publications JA No. 59-82380, JA No. 59-101937 and JA No. 59-101938 (Kitamura, Veno and Nomura) described the treatment of various polymers with many pulses from moderately high energy lasers for the purpose of increasing adhesion and forming a barrier layer to prevent plasticizer migration from within certain polymers.
Bishop and Dyer, Applied Physics Letters, 47, 1229 (1985) extended the photoablation etching work of others to actually cutting through or slitting the polymer film by increasing the energy density of the laser beam by focusing techniques.
The authors of the above references were studying the photodecomposition or photoablation process of UV radiation on polymer surfaces, without regard to whether the polymer was semi-crystalline or amorphous. The present invention does not produce substantial photodecomposition and little or no photoablation, and is concerned only with changes in semicrystalline polymer surface layers produced by exposure to an energy regime different from those used in the prior art.
"Polymer Interface and Adhesion", Souheng Wu, published by Marcel Dekker, Inc., N.Y. and Basel, Chapter 5, page 206 indicates that when a polymer melt cools and solidifies, an amorphous surface is usually formed, although its bulk phase may be semicrystalline. This is at least in part a result of the rejection to the surface of fractions or materials which are not readily accommodated in the crystalline structure. This amorphous surface is believed to be extremely thin, corresponding to only a few layers of molecules, is of the order of no more than 2 or 3 nm, and is generally less than 2 nm is thickness.
U.K. Pat. No. 1,579,002 discloses vacuum glow discharge treatment of polymeric surfaces to increase adhesion to that surface. The glow discharge in the vacuum reduces by 75 to 80% the yellowing typically resulting from corona discharge treatment. During glow discharge treatment, the surfaces are heated to a temperature below the glass transition temperature or melting point.
U.S. Pat. 3,081,485 describes a process for heating and softening polymeric materials using e-beam irradiation so that further mechanical treatment such as stretching and coating can be carried out. The energy densities used (e.g., column 2, line 15) are about two orders of magnitude higher than the energy densities used in the present invention. The authors note on column 2, lines 26 ff. that small traces of irradiated material are evaporated during irradiation. Although the patent describes surface heating, the immediate depth of e-beam penetration (see column 3) appears to be greater than 150 microns. This form of energy would have equal effects on the bulk polymer and would not cause only surface modifications.
U.S. Pat. No. 4,631,155 describes the surface modification of polymers by subjecting a surface to at least one pulse of intense electromagnetic radiation. The surface is disoriented during the relatively long exposure to radiation. Disorientation is indicative of an amorphous surface. Very thick amorphous layers appear to be formed as indicated by the chloroform test described in column 5.
U.S. Pat. No. 3,607,354 discloses the use of highly active hydroxybenzene solvents to delustre the surface of an oriented polyethylene terephthalate film. The solvent acts to dissolve and swell the PET and remains in the surface layer. The chemical composition of the surface layer is different from that of the bulk polymer because of the presence of the very active solvents and the apparent formation of large spherulitic crystallites that tend to interfere with light transmission and contribute to the delustering.